Add __assertion_handler header

BUILD.llvm-raw

@@ -1,6 +1,7 @@
 package(default_visibility = ["//visibility:public"])
 
 load("@rules_pkg//pkg:mappings.bzl", "pkg_files", "strip_prefix")
+load("@bazel_skylib//rules:copy_file.bzl", "copy_file")
 
 genrule(
     name = "gen_config_site",
@@ -24,6 +25,12 @@ genrule(
 #endif // _LIBCPP___CONFIG_SITE' > $@""",
 )
 
+copy_file(
+    name = "write_assertion_handler",
+    src = "libcxx/vendor/llvm/default_assertion_handler.in",
+    out = "libcxx/include/__assertion_handler",
+)
+
 # TODO: gen module.modulemap
 # See https://github.com/llvm/llvm-project/blob/9eeb0293e27c0f1b44304fd9681dcd88fd9edc0a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn#L62-L72
 
@@ -33,6 +40,7 @@ pkg_files(
         "libcxx/include/CMakeLists.txt",
         "libcxx/include/__config_site.in",
     ]) + [
+        "libcxx/include/__assertion_handler",
         "libcxx/include/__config_site",
     ],
     strip_prefix = "libcxx/include",